Dip-forming latex, dip-forming composition and dip-formed article

ABSTRACT

A dip-forming latex obtained by copolymerization of (a) 50-89.5 weight parts of a conjugated diene monomer, (b) 10-40 weight parts of an ethylenically unsaturated nitrile monomer, (c) 0.5-10 weight parts of an ethylenically unsaturated acid monomer and (d) 0-20 weight parts of other copolymerizable ethylenically unsaturated monomer (the total of these monomers is 100 weight parts), wherein the copolymerization is initiated with a monomer mixture comprising at least 80 wt. % of (a), at least 50 wt. % of (b), 10-90 wt. % of (c) and least 80 wt. % of (d), and thereafter, the remainders of monomers are added to a polymerization system to continue copolymerization. This latex gives a dip-formed article exhibiting good softness of touch, high tensile strength and good retention of close fittingness.

TECHNICAL FIELD

This invention relates to a dip-forming latex, a dip-formingcomposition, and a dip-formed article. More particularly, it relates toa dip-forming latex made by copolymerization of a conjugated dienemonomer, an ethylenically unsaturated monomer and an ethylenicallyunsaturated acid monomer; a dip-forming composition comprising thedip-forming latex; and a dip-formed article made by dip-forming thedip-forming composition.

The dip-formed article exhibits good softness of touch, high tensilestrength and preferably good retention of tight fitness, and is usefulas, for example, gloves.

BACKGROUND ART

Rubber gloves are widely used for household uses, industrial uses in,for example, food industry and electronic part industry, and surgicaland other medical uses. It is generally required for rubber gloves that(1) they have good softness of touch and are well-fitting andcomfortable to wear, namely, they are capable of being easily stretchedin conformity with movement of fingers so that fatigue does not itselffelt even when they are worn for long hours, (2) they are not easilybroken, namely, they have a high tensile strength, and (3) they exhibitgood retention of close fittingness, namely, they are not easilyslackened nor crumpled when fingers are moved, and they keep awell-fitted state for a long time.

Rubber gloves made by dip-forming natural rubber latex have widely beenused, but, allergies to natural rubber sometimes cause rashes or itchingdue to protein contained in a slight amount in natural rubber.

Rubber gloves made by dip-forming a synthetic rubber latex, for example,an acrylonitrile-butadiene copolymer latex, are known. Allergy does notdevelop to rubber gloves made from an acrylonitrile-butadiene copolymerlatex, but these gloves exhibit poor balance between the softness oftouch or comfortable fittingness, and the tensile strength.

For example, U.S. Pat. No. 5,014,362 discloses gloves dip-formed from acomposition comprising a carboxyl-modified acrylonitrile-butadienecopolymer latex having incorporated therein minor amounts of zinc oxide,sulfur and a vulcanization accelerator, which are characterized asexhibiting a tensile stress retention of almost zero % as expressed bythe formula of (A/B)×100 wherein B is tensile stress as measuredimmediately after 100% elongation and A is tensile stress as measuredwhen 6 minutes elapses from the measurement of B. These gloves areeasily stretched, well-fitting and comfortable to wear, but theirretention of close fittingness is poor.

International publication WO 97/48765 discloses gloves dip-formed from acomposition comprising a carboxyl-modified acrylonitrile-butadienecopolymer latex, ammonium casein, sulfur and a vulcanizationaccelerator, and not containing zinc oxide. These gloves have hightensile strength, but they are not easily stretched and do not exhibitcomfortable fittingness, and their retention of close fittingness ispoor.

U.S. Pat. No. 5,910,533 discloses a dip-formed article made from acopolymer latex prepared by copolymerization of 80 to 99% by weight of aconjugated diene monomer, 0 to 10% by weight of an unsaturated acidmonomer and 0 to 20% by weight of other unsaturated monomers such asacrylonitrile and methyl methacrylate. As a specific example of thecopolymer latex, a copolymer latex comprised of 87 weight parts ofbutadiene, 10 parts of acrylonitrile and 3 weight parts of methacrylicacid is mentioned. Gloves made from this copolymer latex can easily bestretched, have good softness of touch and are comfortable to wear, butthe tensile strength is low and they are liable to be broken duringwearing.

International publication WO 00/21451 discloses gloves made bydip-forming a composition comprising an acrylonitrile-butadienecopolymer latex containing a specific amount of a carboxyl group, anextremely slight amount of zinc oxide, sulfur and a vulcanizationaccelerator, which are characterized as exhibiting a tensile stressretention in a range of 50 to 70%. These gloves exhibit good retentionof close fittingness, but the balance between the softness of touch orcomfortable fittingness and the tensile strength is occasionally poor.

DISCLOSURE OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide a dip-formed article having good softness of touch andcomfortable fittingness, and high tensile strength.

Another object of the present invention is to provide a dip-formedarticle having good softness of touch, comfortable fittingness, hightensile strength, and an enhanced retention of close fittingness.

Other objects of the present invention are to provide a dip-formingcomposition giving the above-mentioned dip-formed article; and adip-forming latex used for the dip-forming composition.

In one aspect of the present invention, there is provided a dip-forminglatex obtained by copolymerization of 50 to 89.5 parts by weight of aconjugated diene monomer, 10 to 40 parts by weight of an ethylenicallyunsaturated nitrile monomer, 0.5 to 10 parts by weight of anethylenically unsaturated acid monomer and 0 to 20 parts by weight ofother copolymerizable ethylenically unsaturated monomer, provided thatthe total of these monomers is 100 parts by weight, wherein saidcopolymerization is initiated with a monomer mixture comprising at least80% by weight of the amount of conjugated diene monomer used, at least50% by weight of the amount of ethylenically unsaturated nitrile monomerused, 10 to 90% by weight of the amount of ethylenically unsaturatedacid monomer used and at least 80% by weight of the amount of othercopolymerizable ethylenically unsaturated monomer used, and thereafter,the remainders of monomers are added to a polymerization system tocontinue copolymerization.

In another aspect of the present invention, there is provided adip-forming composition comprising the above-mentioned dip-forminglatex.

In another aspect of the present invention, there is provided adip-formed article made by dip-forming the above-mentioned dip-formingcomposition.

BEST MODE FOR CARRYING OUT THE INVENTION

The dip-forming latex of the present invention is obtained bycopolymerization of 50 to 89.5 parts by weight of a conjugated dienemonomer, 10 to 40 parts by weight of an ethylenically unsaturatednitrile monomer, 0.5 to 10 parts by weight of an ethylenicallyunsaturated acid monomer and 0 to 20 parts by weight of othercopolymerizable ethylenically unsaturated monomer, provided that thetotal of these monomers is 100 parts by weight. The copolymerization iscarried out in a manner such that the copolymerization is initiated witha monomer mixture comprising at least 80% by weight of the amount ofconjugated diene monomer used, at least 50% by weight of the amount ofethylenically unsaturated nitrile monomer used, 10 to 90% by weight ofthe amount of ethylenically unsaturated acid monomer used and at least80% by weight of the amount of other copolymerizable ethylenicallyunsaturated monomer used, and thereafter, the remainders of monomers areadded to a polymerization system to continue copolymerization.

In a first preferable embodiment of the dip-forming latex, thedip-forming latex is obtained by a copolymerization procedure such thatthe copolymerization is initiated with a monomer mixture comprising atleast 80% by weight of the amount of conjugated diene monomer used, atleast 80% by weight of the amount of ethylenically unsaturated nitrilemonomer used, 10 to 90% by weight of the amount of ethylenicallyunsaturated acid monomer used and at least 80% by weight of the amountof other copolymerizable ethylenically unsaturated monomer used, andthereafter, the remainders of monomers are added to a polymerizationsystem to continue copolymerization. Preferably, after thecopolymerization of the monomer mixture is initiated, the remainder ofethylenically unsaturated acid monomer is added while the polymerizationconversion of the total monomers is within a range of 5 to 90%, and theremainders of conjugated diene monomer, ethylenically unsaturatednitrile monomer and other copolymerizable ethylenically unsaturatedmonomer are added at any time before the termination ofcopolymerization. This first embodiment of the dip-forming latex ishereinafter referred to “first dip-forming latex” when appropriate. Thefirst dip-forming latex gives a dip-formed article having good softnessof touch, comfortable fittingness, and high tensile strength.

In a second preferable embodiment of the dip-forming latex, thedip-forming latex is obtained by a copolymerization procedure such thatthe copolymerization is initiated with a monomer mixture comprising atleast 80% by weight of the amount of conjugated diene monomer used, 50to 90% by weight of the amount of ethylenically unsaturated nitrilemonomer used, 40 to 90% by weight of the amount of ethylenicallyunsaturated acid monomer used and at least 80% by weight of the amountof other copolymerizable ethylenically unsaturated monomer used, andthereafter, the remainders of ethylenically unsaturated nitrile monomerand ethylenically unsaturated acid monomer are added while thepolymerization conversion of the total monomers is within a range of 5to 95%, and the remainders of conjugated diene monomer and othercopolymerizable ethylenically unsaturated monomer are added at any timebefore the termination of copolymerization. This second embodiment ofthe dip-forming latex is hereinafter referred to “second dip-forminglatex” when appropriate. The second dip-forming latex gives a dip-formedarticle having an enhanced retention of close fittingness, as well asgood softness of touch, comfortable fittingness and high tensilestrength.

By the term “the polymerization conversion of the total monomers added”as used in this specification, we mean the ratio (A/B) of the amount (A)of total monomers which have been converted to a copolymer to the amount(B) of total monomers which have been added to a polymerization system.For example, in the case when, after the initiation of polymerization ofan initially charged monomer mixture, a first part of the remainder of amonomer is added to a polymerization system, the above-mentioned term asof the addition of the first part of monomer means the polymerizationconversion of the monomers in the initially charged monomer mixture. Inthe case when, after the initiation of polymerization of an initiallycharged monomer mixture and further after addition of the first part ofthe remainder of a monomer, a second part of the remainder thereof isadded to a polymerization system, the above-mentioned term as of theaddition of the second part of monomer means the polymerizationconversion of the sum of the monomers in the initially charged monomermixture and the first part of the remainder of monomer.

The amount (A) of total monomers which have been converted to acopolymer is determined, for example, by calculation thereof bydeducting the amount of total unreacted monomers from the amount (B) oftotal monomers which have been added to a polymerization system.

The conjugated monomer includes, for example, 1,3-butadiene, isoprene,2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, 1,3-pentadiene andchloroprene. Of these, 1,3-butadiene and isoprene are preferable.1,3-Butadiene is especially preferable. These conjugated diene monomersmay be used either alone or as a combination of at least two thereof.

The amount of conjugated diene monomer is in the range of 50 to 89.5parts by weight, preferably 55 to 84 parts by weight, more preferably 65to 81 parts by weight, and especially preferably 70 to 80 parts byweight, based on 100 parts by weight of the total amount of monomers. Ifthe amount of conjugated diene monomer is too small, the dip-formedarticle is not satisfactory in softness of touch and comfortablefittingness. In contrast, if the amount of conjugated diene monomer istoo large, the dip-formed article has low tensile strength.

As specific examples of the ethylenically unsaturated nitrile monomer,there can be mentioned acrylonitrile, methacrylonitrile, fumaronitrile,α-chloroacrylonitrile and α-cyanoethylacrylonitrile. Of these,acrylonitrile and methacrylonitrile are preferable. Acrylonitrile isespecially preferable. The ethylenically unsaturated nitrile monomer maybe used either alone or as a combination of at least two thereof.

The amount of ethylenically unsaturated nitrile monomer is in the rangeof 10 to 40 parts by weight, preferably 15 to 36 parts by weight, morepreferably 18 to 27 parts by weight, and especially preferably 18 to 24parts by weight, based on 100 parts by weight of the total amount ofmonomers. If the amount of ethylenically unsaturated nitrile monomer istoo small, the dip-formed article has low tensile strength. In contrast,if the amount of ethylenically unsaturated nitrile monomer is too large,the dip-formed article is not satisfactory in softness of touch andcomfortable fittingness.

The ethylenically unsaturated acid monomer includes ethylenicallyunsaturated monomers having an acidic group such as a carboxyl group, asulfonic acid group or an acid anhydride group. As specific examples ofthe ethylenically unsaturated acid monomer, there can be mentionedethylenically unsaturated monocarboxylic acid monomers such as acrylicacid and methacrylic acid; ethylenically unsaturated polycarboxylic acidmonomers such as itaconic acid, maleic acid and fumaric acid;ethylenically unsaturated polycarboxylic acid anhydride monomers such asmaleic anhydride and citraconic anhydride; ethylenically unsaturatedsulfonic acid monomers such as styrenesulfonic acid; and ethylenicallyunsaturated polycarboxylic acid partial ester monomers such as monobutylfumarate, monobutyl maleate and mono-2-hydroxypropyl maleate. Of these,ethylenically unsaturated carboxylic acid monomers are preferable.Ethylenically unsaturated monocarboxylic acid monomers are morepreferable. Methacrylic acid is especially preferable. Theseethylenically unsaturated acid monomers may be used in the form of asalt such as an alkali metal salt or an ammonium salt. The ethylenicallyunsaturated acid monomers may be used either alone or as a combinationof at least two thereof.

The amount of ethylenically unsaturated acid monomer is in the range of0.5 to 10 parts by weight, preferably 1 to 9 parts by weight, morepreferably 1 to 8 parts by weight, and especially preferably 2 to 6parts by weight, based on 100 parts by weight of the total amount ofmonomers. If the amount of ethylenically unsaturated acid monomer is toosmall, the dip-formed article has low tensile strength. In contrast, ifthe amount of ethylenically unsaturated acid monomer is too large, thedip-formed article is not satisfactory in softness of touch, comfortablefittingness, and retention of close fittingness.

As specific examples of the other copolymerizable ethylenicallyunsaturated monomers which are optionally used, there can be mentionedvinyl aromatic monomers such as styrene, alkylstyrenes and vinylnaphthalene; fluoroalkyl vinyl ether monomers such as fluoroethyl vinylether; ethylenically unsaturated amide monomers such as acrylamide,N-methylolacrylamide, N,N-dimethylolacrylamide,N-methoxymethylacrylamide, N-propoxymethylacrylamide, methacrylamide,N-methylol-methacrylamide, N,N-dimethylolmethacrylamide,N-methoxymethylmethacrylamide and N-propoxymethyl-methacrylamide;ethylenically unsaturated carboxylic acid ester monomers such as methylacrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate,trifluoroethyl acrylate, tetrafluropropyl acrylate, methoxymethylacrylate, ethoxyethyl acrylate, methoxyethoxyethyl acrylate, cyanomethylacrylate, 2-cyanoethyl acrylate, 1-cyanopropyl acryalte,2-ethyl-6-cyanohexyl acrylate, 3-cyanopropyl acrylate, hydroxyethylacrylate, hydroxypropyl acrylate, glycidyl acrylate, dimethylaminoethylacrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate,2-ethylhexyl methacrylate, trifluoroethyl methacrylate, tetrafluropropylmethacrylate, methoxymethyl methacrylate, ethoxyethyl methacrylate,methoxyethoxyethyl methacrylate, cyanomethyl methacrylate, 2-cyanoethylmethacrylate, 1-cyanopropyl methacryalte, 2-ethyl-6-cyanohexylmethacrylate, 3-cyanopropyl methacrylate, hydroxyethyl methacrylate,hydroxypropyl methacrylate, glycidyl methacrylate, dimethylaminoethylmethacrylate, dibutyl maleate, dibutyl fumarate and diethyl maleate; andcrosslinking monomers such as divinylbenzene, polyethylene glycoldiacrylate, polyethylene glycol dimethacrylate, polypropylene glycoldiacrylate, polypropylene glycol dimethacrylate, trimethylolpropanetriacrylate, trimethylolpropane trimethacrylate, pentaerithritolacrylate and pentaerithritol methacrylate. These optional ethylenicallyunsaturated monomers may be used either alone or as a combination of atleast two thereof.

The amount of the optional ethylenically unsaturated monomer is notlarger than 20 parts by weight, preferably not larger than 15 parts byweight, more preferably not larger than 10 parts by weight, andespecially preferably not larger than 8 parts by weight, based on 100parts by weight of the total amount of monomers. If the amount of theoptional ethylenically unsaturated monomer is too large, the balancebetween the softness of touch and comfortable fittingness, and thetensile strength is poor.

The dip-forming latex of the present invention is obtained bycopolymerization of the above-mentioned monomers, preferably by anemulsion copolymerization procedure.

In the copolymerization for the production of dip-forming latex, thetime at which the monomers are added to a polymerization system isimportant. That is, the copolymerization is initiated with a monomermixture comprising at least 80% by weight of the amount of conjugateddiene monomer used, at least 50% by weight of the amount ofethylenically unsaturated nitrile monomer used, 10 to 90% by weight ofthe amount of ethylenically unsaturated acid monomer used and at least80% by weight of the amount of other copolymerizable ethylenicallyunsaturated monomer used, and thereafter, the remainders of monomers areadded to a polymerization system to continue copolymerization.

For the production of the above-mentioned first dip-forming latex, thecopolymerization is initiated with a monomer mixture comprising at least80% by weight of the amount of conjugated diene monomer used, at least80% by weight of the amount of ethylenically unsaturated nitrile monomerused, 10 to 90% by weight of the amount of ethylenically unsaturatedacid monomer used and at least 80% by weight of the amount of othercopolymerizable ethylenically unsaturated monomer used, and thereafter,the remainders of monomers are added to a polymerization system tocontinue copolymerization. Preferably, after the copolymerization of themonomer mixture is initiated, the remainder of ethylenically unsaturatedacid monomer is added while the polymerization conversion of the totalmonomers added is within a range of 5 to 90%, and the remainders ofconjugated diene monomer, ethylenically unsaturated nitrile monomer andother copolymerizable ethylenically unsaturated monomer are added at anytime before the termination of copolymerization.

For the production of the above-mentioned second dip-forming latex, thecopolymerization is initiated with a monomer mixture comprising at least80% by weight of the amount of conjugated diene monomer used, 50 to 90%by weight of the amount of ethylenically unsaturated nitrile monomerused, 40 to 90% by weight of the amount of ethylenically unsaturatedacid monomer used and at least 80% by weight of the amount of othercopolymerizable ethylenically unsaturated monomer used, and thereafter,the remainders of ethylenically unsaturated nitrile monomer andethylenically unsaturated acid monomer are added while thepolymerization conversion of the total monomers added is within a rangeof 5 to 95%, and the remainders of conjugated diene monomer and othercopolymerizable ethylenically unsaturated monomer are added at any timebefore the termination of copolymerization.

The ethylenically unsaturated nitrile monomer is initially added in apolymerization vessel in an amount of at least 50% by weight of itstotal amount used for polymerization, and, after the initiation ofpolymerization, the remainder thereof is added to continuecopolymerization.

More specifically, for the production of the first dip-forming latex,the ethylenically unsaturated nitrile monomer is initially added in apolymerization vessel preferably in an amount of at least 80% by weight,more preferably at least 90% and especially preferably 100% of its totalamount used for polymerization, and, after the initiation ofpolymerization, the remainder thereof is added to continuecopolymerization.

For the production of the second dip-forming latex, the ethylenicallyunsaturated nitrile monomer is initially added in a polymerizationvessel preferably in an amount of 50 to 90% by weight, more preferably55 to 85% by weight and especially preferably 60 to 85% by weight of itstotal amount used for polymerization. If the amount of ethylenicallyunsaturated nitrile monomer initially added in a polymerization vesselis too small, the dip-formed article has low tensile strength. Incontrast, if the amount of ethylenically unsaturated nitrile monomerinitially added is too large, the balance among the softness of touch,comfortable fittingness, tensile strength and retention of closefittingness tends to be poor.

After the initiation of polymerization for the production of the seconddip-forming latex, the remainder of ethylenically unsaturated nitrilemonomer is added while the polymerization conversion of the totalmonomers added is preferably within a range of 5 to 95%, more preferably10 to 90% by weight and especially preferably 20 to 90% by weight. Afterthe initiation of polymerization, if the remainder of ethylenicallyunsaturated nitrile monomer is added while the polymerization conversionof the total monomers added is too small, the dip-formed article tendsto have poor softness of touch, poor fittingness and low tensilestrength. In contrast, if the remainder of ethylenically unsaturatednitrile monomer is added while the polymerization conversion of thetotal monomers added is too large, the dip-formed article tends to havelow tensile strength. The remainder of ethylenically unsaturated nitrilemonomer is added preferably while the polymerization conversion of theethylenically unsaturated nitrile monomer added is in the range of 40 to95% by weight, more preferably 45 to 92% by weight and especiallypreferably 45 to 85% by weight for enhancement of tensile strength.

After the initiation of polymerization, the remainder of ethylenicallyunsaturated nitrile monomer is added preferably at two or more times. Inthis case, the amount of ethylenically unsaturated nitrile monomer addedin each time is preferably equal to each other for more enhancement ofbalance between the softness of touch and comfortable fittingness, andthe tensile strength. The number of times for the addition ofethylenically unsaturated nitrile monomer is not particularly limited,and may be infinite. That is, the remainder of ethylenically unsaturatednitrile monomer can be continuously added.

The ethylenically unsaturated acid monomer is initially added in apolymerization vessel in an amount of 10 to 90% by weight of its totalamount used for polymerization, and, after the initiation ofpolymerization, the remainder thereof is added to continuecopolymerization.

More specifically, for the production of the first dip-forming latex,the ethylenically unsaturated acid monomer is initially added in apolymerization vessel in an amount of 10 to 90% by weight, preferably 30to 85% by weight and more preferably 50 to 80% by weight of its totalamount used for polymerization, and, after the initiation ofpolymerization, the remainder thereof is added to continuecopolymerization. If the amount of ethylenically unsaturated acidmonomer initially added is too small, the dip-formed article has lowtensile strength. In contrast, if the amount thereof initially added istoo large, the dip-formed article is not satisfactory in softness oftouch, comfortable fittingness and tensile strength.

After the initiation of polymerization for the production of the firstdip-forming latex, the remainder of ethylenically unsaturated acidmonomer is added while the polymerization conversion of the totalmonomers added is preferably within a range of 5 to 90%, more preferably20 to 80% by weight and especially preferably 40 to 80% by weight. Whenthe remainder of ethylenically unsaturated monomer is added while thepolymerization conversion of the total monomers added is within thisrange, the dip-formed article has well balanced softness of touch,comfortable fittingness and tensile strength. The remainder ofethylenically unsaturated acid monomer may be added either in one lot,or in two or more lots. It can also be added in a continuous manner. Theaddition in one lot is preferable.

For the production of the second dip-forming latex, the ethylenicallyunsaturated acid monomer is initially added in a polymerization vesselin an amount of 40 to 90% by weight, preferably 50 to 85% by weight andmore preferably 60 to 80% by weight of its total amount used forpolymerization. If the amount of ethylenically unsaturated acid monomerinitially added in a polymerization vessel is too small, the dip-formedarticle has poor tensile strength and poor retention of tight fitness.In contrast, if the amount of ethylenically unsaturated acid monomerinitially added is too large, the dip-formed article is not satisfactoryin softness of touch, comfortable fittingness and tensile strength.

After the initiation of polymerization for the production of the seconddip-forming latex, the remainder of ethylenically unsaturated acidmonomer is added while the polymerization conversion of the totalmonomers added is within a range of 5 to 95% by weight, preferably 10 to90% by weight, more preferably 20 to 80% by weight and especiallypreferably 40 to 70% by weight. If the remainder of ethylenicallyunsaturated acid monomer is added while the polymerization conversion ofthe total monomers added is within this range, the dip-formed articlehas well-balanced and good softness of touch, comfortable fittingnessand tensile strength. The remainder of ethylenically unsaturated acidmonomer may be added either in one lot, or in two or more lots. It canalso be added in a continuous manner. The addition in one lot ispreferable.

The conjugated diene monomer is initially added in a polymerizationvessel in an amount of at least 80% by weight, preferably at least 90%by weight of its total amount used for polymerization, and, after theinitiation of polymerization, the remainder thereof is added to continuecopolymerization. Preferably, the entire amount of conjugated dienemonomer used for polymerization is initially added before the initiationof polymerization.

The optional other copolymerizable ethylenically unsaturated monomer isinitially added in a polymerization vessel in an amount of at least 80%by weight, preferably at least 90% by weight of its total amount usedfor polymerization, and, after the initiation of polymerization, theremainder thereof is added to continue copolymerization. Preferably, theentire amount of the monomer used for polymerization is initially addedbefore the initiation of polymerization.

The procedures for copolymerization may be conventional provided thatthe time at which the monomers are added to a polymerization system issatisfied with the above requirements. For example, in the case of anemulsion polymerization, a monomer mixture is polymerized by using apolymerization initiator in the presence of water and an emulsifier,and, when the polymerization conversion reaches a predetermined value, apolymerization stopper is added to terminate polymerization.

The emulsifier used for emulsion copolymerization is not particularlylimited, and, as specific examples thereof, there can be mentionednonionic emulsifiers such as polyoxyethylene alkyl ethers,polyoxyethylene alkyl phenol ethers, polyoxyethylene alkyl esters andpolyoxyethylene sorbitan alkyl esters; anionic emulsifiers such as saltsof fatty acids, for example, myristic acid, palmitic acid, oleic acidand linolenic acid, alkylbenzenesulfonic acid salts, for example, sodiumdodecylbenzenesulfonate, and higher alcohol sulfuric acid ester saltsand alkylsulfosuccinic acid salts; cationic emulsifiers such asalkyltrimethylammonium chloride, dialkylammonium chloride andbenzylammonium chloride; and copolymerizable emulsifiers such assulfoesters of α,β-unsaturated carboxylic acids, sulfate esters ofα,β-unsaturated carboxylic acids and sulfoalkyl aryl ethers. Of these,anionic emulsifiers are preferable. These emulsifiers may be used eitheralone or as a combination of at least two thereof. The amount ofemulsifier is in the range of 0.1 to 10 parts by weight based on 100parts by weight of the total monomers added.

The amount of water used for emulsion copolymerization is in the rangeof 80 to 500 parts by weight, preferably 100 to 300 parts by weight,based on 100 parts by weight of the total monomers added.

The polymerization initiator used is not particularly limited, and, asspecific examples thereof, there can be mentioned inorganic peroxidessuch as sodium persulfate, potassium persulfate, ammonium persulfate,potassium perphosphate and hydrogen peroxide; organic peroxides such asdiisopropylbenzene hydroperoxide, cumene hydroperoxide, tert-butylhydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide,2,5-dimethylhexane-2,5-dihydroperoxide, di-tert-butyl hydroperoxide,di-α-cumyl peroxide, acetyl peroxide, isobutyryl peroxide and benzoylperoxide; and azo compounds such as azobisisobutyronitrile,azobis-2,4-dimethylvaleronitrile and methyl azobisisobutyrate. Thesepolymerization initiators may be used either alone or as a combinationof at least two thereof. Peroxide polymerization initiators arepreferable because a latex can be stably produced and a dip-formedarticle having enhanced softness of touch and high tensile strength canbe obtained from the latex. The amount of polymerization initiator ispreferably in the range of 0.01 to 1.0 part by weight based on 100 partsby weight of the total monomers added.

The peroxide polymerization initiator can be used in combination with areducing agent, as a redox polymerization initiator. The reducing agentused is not particularly limited and includes, for example, compoundscontaining a metal ion in a reduced state such as ferrous sulfate andcuprous naphthenate; sulfonic acid compounds such as sodiummethanesulfonic acid; and amine compounds such as dimethylaniline. Thesereducing agents may be used either alone or in combination. The amountof reducing agent is preferably in the range of 0.03 to 10 parts byweight based on 1 part by weight of peroxide.

As a polymerization stopper, there can be used, for example,hydroxylamine, hydroxylamine sulfate salt, diethyl hydroxylamine,hydroxylaminesulfonic acid and its alkali metal salts; sodiumdimethyldithiocarbamate, hydroquinone derivatives and catecholderivatives; and aromatic hydroxydithiocarboxylic acids such ashydroxydimethyl-benzenethiocarboxylic acid,hydroxydiethylbenzene-thiocarboxylic acid andhydroxydibutylbenzenethiocarboxylic acid, and alkali metal saltsthereof. The amount of polymerization stopper is not particularlylimited, but is usually in the range of 0.1 to 2 parts by weight basedon 100 parts by weight of the total monomers added.

Polymerization auxiliaries can be used for emulsion copolymerizationaccording to the need, which include, for example, a molecular weightmodifier, a particle size modifier, a chelating agent and an oxygenscavenger.

The polymerization temperature is not particularly limited, but isusually in the range of 0 to 95° C., preferably 5 to 70° C.

The polymerization conversion at which the polymerization reaction isterminated is preferably at least 90%, more preferably at least 93%.

After the polymerization reaction is terminated, unreacted monomers areremoved from a polymerization mixture and the solid content and pH valuethereof are adjusted according to the need to give a desired copolymerlatex.

Additives such as an antioxidant, a preservative, an anti-fungus agentand a dispersant can be incorporated in the thus-obtained latexaccording to the need.

The copolymer latex preferably has a number average particle diameter inthe range of 60 to 300 nm, more preferably 80 to 150 nm. The particlediameter can be adjusted to a desired value by varying the amounts of anemulsifier and polymerization initiator.

The dip-forming composition of the present invention comprises theabove-mentioned dip-forming latex.

Preferably a vulcanizing agent and a vulcanization accelerator areincorporated in the dip-forming composition of the present invention. Ifdesired, zinc oxide can be further incorporated in the dip-formingcomposition.

As the vulcanizing agent, those which are conventionally used indip-forming are mentioned. As specific examples thereof, there can bementioned sulfur such as powdery sulfur, flower of sulfur, precipitatedsulfur, colloidal sulfur, surface-treated sulfur and insoluble sulfur;and polyamines such as hexamethylenediamine, triethylenetetramine andtetraethylenepentamine. Of these, sulfur is preferable.

The amount of vulcanizing agent is preferably in the range of 0.5 to 10parts by weight, more preferably 2 to 5 parts by weight and especiallypreferably 3.5 to 4.5 parts by weight based on 100 parts by weight ofthe solid content of latex.

As the vulcanization accelerator, those which are conventionally used indip-forming are mentioned. As specific examples thereof, there can bementioned dithiocarbamic acids such as diethyldithiocarbamic acid,dibutyldithiocarbamic acid, di-2-ethylhexyldithiocarbamic acid,dicyclohexyldithio-carbamic acid, diphenyldithiocarbamic acid anddibenzyldithiocarbamic acid, and zinc salt thereof; and2-mercaptobenzothiazole, zinc salt of 2-mercaptobenzothiazole,2-mercaptothiazoline, dibenzothiazyldisulfide,2-(2,4-dinitrophenylthio)benzothiazole,2-(N,N-diethylthio-carbaylthio)benzothiazole,2-(2,6-dimethyl-4-morpholinothio)benzothiazole,2-(4′-morpholino-dithio)benzothiazole, 4-morphonylyl-2-benzothiazyldisulfide and 1,3-bis(2-benzothiazyl-mercaptomethyl)urea. Of these, zincdibutyldithiocarbamate, 2-mercaptobenzothiazole and zinc salt of2-mercaptobenzothiazole are preferable. These vulcanization acceleratorsmay be used either alone or as a combination of at least two thereof.

The amount of vulcanizing accelerator is preferably in the range of 0.1to 10 parts by weight, more preferably 0.5 to 5 parts by weight andespecially preferably 1 to 3 parts by weight based on 100 parts byweight of the solid content of latex.

The amount of zinc oxide is preferably not larger than 5 parts byweight, more preferably not larger than 1 part by weight and especiallypreferably not larger than 0.5 part by weight based on 100 parts byweight of the solid content of latex.

Conventional ingredients such as a pH adjuster, a thickener, anantioxidant, a dispersant, a pigment, a filler and a softener may beincorporated in the dip-forming composition of the present invention,according to the need. Provided that the object of the present inventionis achieved, other latex such as natural rubber latex or isoprene rubberlatex can be incorporated with the dip-forming latex.

The dip-forming composition of the present invention has a solid contentin the range of 20 to 40% by weight, preferably 25 to 35% by weight.

The dip-forming composition of the present invention has a pH value inthe range of 8.5 to 12, preferably 9 to 11.

The dip-formed article of the present invention is made by dip-formingthe above-mentioned dip-forming composition. A conventional dip-formingmethod can be adopted, which includes, for example, a direct dip-formingmethod, an anode cohesion dip-forming method, a Teague cohesiondip-forming method and a combination of these methods. Of these, ananode cohesion dip-forming method is preferable because a dip-formedarticle having a uniform thickness is easily obtained.

The anode cohesion dip-forming method is carried out by a processcomprising the steps of dipping a dip-forming form in a solution of acoagulant to form a layer comprised of the coagulant solution on theform; and dipping the form having the coagulating solution layer thereonin a dip-forming composition to form a coagulated layer comprised of thedip-forming composition.

As specific examples of the coagulant, there can be mentioned metalhalides such as barium chloride, calcium chloride, magnesium chloride,zinc chloride and aluminum chloride; nitric acid salts such as bariumnitrate, calcium nitrate and zinc nitrate; acetic acid salts such asbarium acetate, calcium acetate and zinc acetate; and sulfuric acidsalts such as calcium sulfate, magnesium sulfate and aluminum sulfate.Of these, calcium chloride and calcium nitrate are preferable.

The coagulant is usually used as a solution in water, an alcohol or amixture thereof. The concentration of coagulant in the solution isusually in the range of 5 to 70% by weight, preferably 20 to 50% byweight.

The coagulated layer of the dip-forming composition, formed on thesurface of a dip-forming form is usually heat-treated to cure.

The form having formed thereon the coagulated dip-forming compositionlayer can be dipped in water, preferably warm water maintained at atemperature of 30 to 70° C., for 1 to 60 minutes to remove water-solubleimpurities such as, for example, excessive emulsifier and coagulant,from the coagulated dip-forming composition layer. This water washingcan be carried out either before or after the heat-treatment of thecoagulated dip-forming composition layer, but, the water washing ispreferably carried out before the heat-treatment because water-solubleimpurities can be more effectively removed.

The water-washed coagulated composition layer is heat-treated usually ata temperature of 100 to 150° C. for 10 to 120 minutes to cure thecoagulated composition layer. The heating can be carried out by anexternal heating method using infrared rays or heated air, or aninternal heating method using high frequency. Of these, an externalheating method using heated air is preferable.

The cured, coagulated dip-forming composition layer is released from theform to obtain a dip-formed article. The release can be carried outmanually or by applying water pressure or compressed air.

After the release from the form, the dip-formed article can be furtherheat-treated at a temperature of 60 to 120° C. for 10 to 120 minutes.

The dip-formed article may have a surface-treated layer formed on theinner surface and/or the outer surface.

By using the dip-forming latex of the present invention, a dip-formedarticle having a tensile stress at 300% elongation of not larger than2.5 MPa, a tensile strength of at least 15 MPa, preferably at least 20MPa, and a tensile stress retention of at least 70%, preferably largerthan 70%, as measured when 6 minutes elapses from the time of 100%elongation, can be easily obtained. Further, a dip-formed article havingthese characteristics and a swelling degree in methyl ethyl ketone(hereinafter abbreviated to “MEK” when appropriate) of not larger than200%, preferably not larger than 180%, can also be obtained. The smallerthe swelling degree in MEK, the more excellent the oil resistance.

EXAMPLES

The invention will now be described by the following examples wherein %and parts are by weight unless otherwise specified.

Polymerization conversion of acrylonitrile in a polymerization mixtureand properties of a dip-formed article were evaluated by the followingmethods.

Polymerization Conversion of Acrylonitrile (AN) (%)

A part of a polymerization liquid was taken, and the content ofunreacted acrylonitrile (AN) was measured. From the amount (a) of ANinitially charged and the measured content (b) of unreacted AN, theratio (%) ([(a−b)/a]×100) of the amount of AN (a−b) converted to acopolymer to the amount (a) of AN initially charged was calculated. Inthe case when, after initiation of polymerization of an initiallycharged monomer mixture and further after addition of a first part ofthe remainder of AN, a second part of the remainder of AN is added, thepolymerization conversion of AN as of the addition of the second partthereof is calculated from the formula:Polymerization conversion of AN (%)=[(a+a′)−b]/(a+a′)×100where a: amount of AN in initially charged monomer mixture

-   -   a′: amount of first part of the remainder of AN    -   b: amount of unreacted AN

Tensile Stress at 300% Elongation (MPa)

A dumbbell specimen (Die-C) was punched out from a dip-formed article ofa glove form, according to ASTM D412.

Tensile stress at 300% elongation was measured on the dumbbell specimenat a drawing rate of 500 mm/min by Tensilon tensile tester (“RTC-1225A”availavle from Orientec K.K.). The smaller the tensile stress at 300%elongation, the more excellent the softness of touch and comfortablefittingness of dip-formed article.

Tensile Strength (MPa)

Tensile strength was measured on the dumbbell specimen at a drawing rateof 500 mm/min by Tensilon tensile tester (the same as mentioned above)immediately before breaking.

Elongation at Break (%)

Elongation at break was measured on the dumbbell specimen at a drawingrate of 500 mm/min by Tensilon tensile tester (the same as mentionedabove) immediately before breaking.

Tensile Stress Retention (%)

Tensile stress was measured on the dumbbell specimen by Tensilon tensiletester (the same as mentioned above). Tensile stress retention wasdetermined from the tensile stress (Md0) as measured immediately afterthe elongation reached 100%, and the tensile stress (Md6) as measuredwhen the specimen at the elongation of 100% was kept as it was for 6minutes. The tensile stress retention (%) was defined as the ratio (%)of Md0/Md6×100. The larger the tensile stress retention, the moreexcellent the retention of close fittingness of dip-formed article.

Swelling Degree in Methyl Ethyl Ketone (MEK) (%)

A disc specimen having a diameter (D1) of 2 cm was punched out from adip-formed article. The specimen was dipped in a MEK bath having a largevolume at 20° C. for 72 hours, to be thereby swelled. Diameter (D2) wasmeasured after swelling, and the swelling degree was calculated from thefollowing equation (1).Swelling Degree in MEK (%)=(D2/D1)²×100  (1)

Example 1

A pressure polymerization vessel was charged with 18 parts ofacrylonitrile, 3 parts of methacrylic acid, 74 parts of 1,3-butadiene,0.3 part of tert-dodecyl mercaptan as a molecular weight modifier, 150parts of deionized water, 2.5 parts of sodium dodecylbenzenesulfonate,0.2 part of potassium persulfate and 0.1 part of sodiumethylenediaminetetraacetate. Then the temperature of the content waselevated to 39° C. to initiate polymerization.

When the polymerization conversion of the total monomers added reached60% (at this time, the polymerization conversion of acrylonitrilereached 66%), 4 parts of acrylonitrile and 1 part of methacrylic acidwere added to a polymerization system. While the temperature wasmaintained at 39° C., the polymerization was continued until thepolymerization conversion reached 95%. Thereafter 0.1 part ofdiethylhydroxlamine was added to terminate the polymerization.

Unreacted monomers were distilled off from the thus-prepared copolymerlatex, and then, the solid content and pH value of the latex wereadjusted to give a copolymer latex A having a solid content of 45% and apH value of 8.5.

An aqueous dispersion of a vulcanizing agent was prepared by mixingtogether 3.5 parts of sulfur, 0.1 part of zinc oxide, 2 parts of zincdibutylcarbamate, 0.03 part of potassium hydroxide and 5.63 parts ofwater. 11.26 parts of the aqueous dispersion of a vulcanizing agent wasmixed with 250 parts of the above-mentioned copolymer latex (solidcontent: 100 parts), and then, deionized water was added to the mixtureto prepare a dip-forming composition having a solid content of 30%.

An aqueous coagulant solution was prepared by mixing together 20 partsof calcium nitrate, 0.05 part of polyoxyethyleneoctyl phenyl ether(nonionic emulsifier) and 80 parts of water. A dip-forming glove formwas dipped in the aqueous coagulant solution for 1 minute, and then, theglove form was taken out and dried at 50° C. for 3 minutes whereby thecoagulant was deposited on the glove form.

The glove form having the coagulant deposited thereon was dipped in theabove-mentioned dip-forming composition for 6 minutes. The glove formwas taken out from the dip-forming composition, and then, the glove formhaving thereon a dip-formed layer was dried at 25° C. for 3 minutes andthen dipped in warm water at 40° C. for 3 minutes to removewater-soluble impurities. Then the glove form was dried at 80° C. for 20minutes and subsequently heat-treated at 120° C. for 25 minutes wherebythe dip-formed layer was vulcanized. Finally the vulcanized, dip-formedlayer was peeled from the glove form to obtain a dip-formed article of aglove shape. Properties of the dip-formed article were evaluated. Theresults are shown in Table 1.

Examples 2 and 3

Copolymer latexes B and C were prepared by the same procedures asdescribed in Example 1 except that the amount of the initial monomermixture charged, the amounts of acrylonitrile and methacrylic acid addedafter the initiation of polymerization, and the conditions under whichthe additional monomers were added were varied as shown in Table 1.

Dip-formed articles were made by the same procedures as described inExample 1 except that copolymer latexes B and C were used instead ofcopolymer latex A. Properties of the dip-formed article were evaluated.The results are shown in Table 1.

Comparative Examples 1 and 2

Copolymer latexes D and E were prepared by the same procedures asdescribed in Example 1 except that the amount of the initial monomermixture charged, the amounts of acrylonitrile and methacrylic acid addedafter the initiation of polymerization, and the conditions under whichthe additional monomers were added were varied as shown in Table 1.

Dip-formed articles were made by the same procedures as described inExample 1 except that copolymer latexes D and E were used instead ofcopolymer latex A. Properties of the dip-formed article were evaluated.The results are shown in Table 1. TABLE 1 Comparative Example Example 12 3 1 2 Monomer Composition (parts) Initial charge: 1,3-butadiene 74 7473 74 74 Methacrylic acid (MA) 3 3 2 4 — Acrylonitrile (AN) 18 15 20 2218 % of initially charged MA to total MA 75 75 66.7 100 0 % of initiallycharged AN to total AN 81.8 68.1 83.3 100 81.8 Amount of AN added afterinitiation of polymerization *1 at polymerization conversion of 40% —3.5(48%) — — — at polymerization conversion of 60% 4(66%) — 4(64%) —4(67%) at polymerization conversion of 70% — 3.5(73%) — — — Amount of MAadded after initiation of polymerization at polymerization conversion of40% — 1 — — — at polymerization conversion of 60% 1 — 1 — 4 CopolymerLatex A B C D E Properties of Dip-Formed Article Tensile stress at 300%elongation (MPa) 1.9 1.8 2 2.5 3.5 Tensile strength (MPa) 23.5 25.2 24.816.7 17.8 Elongation at break (%) 610 630 610 570 510 Retention oftensile stress (%) 75 76 73 67 56 Swelling degree in MEK (%) 144 135 156222 234*1: Percents within parentheses indicate polymerization conversion of ANat which AN was added

The following will be seen from Table 1.

The dip-formed article (Comparative Example 1) made from copolymer latexD prepared by copolymerization of the monomers, the total of which wereinitially charged, exhibited moderately good retention of tensilestress, but had slightly poor softness of touch, and slightly poortensile strength.

The dip-formed article (Comparative Example 2) made from copolymer latexE prepared by copolymerization of the monomers wherein acrylonitrile andmethacrylic acid were added in the midst of polymerization, butmethacrylic acid was not initially charged, had a tensile strength ofapproximately the same magnitude as, but exhibited poor retention oftensile stress and poor softness of touch as compared with that ofComparative Example 1.

In contrast to the comparative examples, dip-formed articles (Examples1-3) made from copolymer latexes A, B and C prepared by copolymerizationof the monomers wherein part of acrylonitrile and part of methacrylicacid were initially charged and the remainders thereof were added in themidst of polymerization, exhibited good softness of touch, high tensilestrength and high retention of tensile stress.

Example 4

A pressure polymerization vessel was charged with 23 parts ofacrylonitrile, 3 parts of methacrylic acid, 73 parts of 1,3-butadiene,0.3 part of tert-dodecyl mercaptan as a molecular weight modifier, 150parts of deionized water, 2.5 parts of sodium dodecylbenzenesulfonate,0.2 part of potassium persulfate and 0.1 part of sodiumethylenediaminetetraacetate. Then the temperature of the content waselevated to 37° C. to initiate polymerization.

When the polymerization conversion of the total monomers added reached60%, 0.1 part of tert-dodecyl mercaptan and 1 part of methacrylic acidwere added to a polymerization system. The temperature was elevated to40° C., and, while the temperature was maintained at 40° C., thepolymerization was continued until the polymerization conversion reached97%. Thereafter 0.1 part of diethylhydroxlamine was added to terminatethe polymerization.

Unreacted monomers were distilled off from the thus-prepared copolymerlatex, and then, the solid content and pH value of the latex wereadjusted to give a copolymer latex F having a solid content of 40% and apH value of 8.5.

An aqueous dispersion of a vulcanizing agent was prepared by mixingtogether 3 parts of sulfur, 0.3 part of zinc oxide, 1.5 parts of zincdibutylcarbamate, 1.5 parts of zinc diethylcarbamate, 0.03 part ofpotassium hydroxide and 6.33 parts of water. 12.66 parts of the aqueousdispersion of a vulcanizing agent was mixed with 250 parts of theabove-mentioned copolymer latex (solid content: 100 parts), and then,deionized water was added to the mixture to prepare a dip-formingcomposition having a solid content of 30%.

An aqueous coagulant solution was prepared by mixing together 20 partsof calcium nitrate, 0.05 part of polyoxyethyleneoctyl phenyl ether(nonionic emulsifier) and 80 parts of water. A dip-forming glove formwas dipped in the aqueous coagulant solution for 1 minute, and then, theglove form was taken out and dried at 50° C. for 3 minutes whereby thecoagulant was deposited on the glove form.

The glove form having the coagulant deposited thereon was dipped in theabove-mentioned dip-forming composition for 6 minutes. The glove formwas taken out from the dip-forming composition, and then, the glove formhaving thereon a dip-formed layer was dried at 25° C. for 3 minutes andthen dipped in warm water at 40° C. for 3 minutes to removewater-soluble impurities. Then the glove form was dried at 80° C. for 20minutes and subsequently heat-treated at 120° C. for 25 minutes wherebythe dip-formed layer was vulcanized. Finally the vulcanized, dip-formedlayer was peeled from the glove form to obtain a dip-formed article of aglove shape. Properties of the dip-formed article were evaluated. Theresults are shown in Table 2.

Example 5

Copolymer latex G was prepared by the same procedures as described inExample 4 except that the composition of the initial monomer mixturecharged, and the amount of methacrylic acid added after the initiationof polymerization were varied as shown in Table 2.

A dip-formed article was made by the same procedures as described inExample 4 except that copolymer latex G was used instead of copolymerlatex F. Properties of the dip-formed article were evaluated. Theresults are shown in Table 2.

Comparative Example 3

Copolymer latex H was prepared by the same procedures as described inExample 4 except that the composition of the initial monomer mixturecharged was varied as shown in Table 2 and methacrylic acid was notadded after the initiation of polymerization.

A dip-formed article was made by the same procedures as described inExample 4 except that copolymer latex H was used instead of copolymerlatex F. Properties of the dip-formed article were evaluated. Theresults are shown in Table 2.

Comparative Example 4

Copolymer latex J was prepared by the same procedures as described inExample 4 except that the composition of the initial monomer mixturecharged and the amount of methacrylic acid added after the initiation ofpolymerization were varied as shown in Table 2.

A dip-formed article was made by the same procedures as described inExample 4 except that copolymer latex J was used instead of copolymerlatex F. Properties of the dip-formed article were evaluated. Theresults are shown in Table 2. TABLE 2 Comparative Example Example 4 5 34 Monomer Composition (parts) Initial charge: 1,3-butadiene 73 71 73 73Acrylonitrile (AN) 23 26 23 23 Methacrylic acid (MA) 3 2 4 — % ofinitially charged MA to total MA 75 67 100 0 Amount of MA added afterinitiation 1 1 — 4 of polymerization Copolymer Latex F G H J Propertiesof Dip-Formed Article Tensile stress at 300% elongation (MPa) 2.0 2.12.3 3.2 Tensile strength (MPa) 22.3 24.1 16.5 15.4 Elongation at break(%) 600 610 590 510

The following will be seen from Table 2.

The dip-formed article (Comparative Example 3) made from copolymer latexH prepared by copolymerization of the monomers, wherein methacrylic acidwas not added after initiation of polymerization, exhibited moderatelygood softness of touch, but had poor tensile strength.

The dip-formed article (Comparative Example 4) made from copolymer latexJ prepared by copolymerization of the monomers wherein methacrylic acidwas not incorporated in the monomer mixture initially charged, exhibitedpoor softness of touch and poor tensile strength.

In contrast to the comparative examples, dip-formed articles (Examples 4and 5) made from copolymer latexes F and G prepared by copolymerizationof the monomers wherein part of methacrylic acid was initially chargedand the remainder thereof was added in the midst of polymerization,exhibited good softness of touch and high tensile strength.

Industrial Applicability

The dip-formed article of the present invention exhibits good softnessof touch and comfortable fittingness, has high tensile strength andpreferably exhibits high retention of close fittingness. This dip-formedarticle can have a thickness of about 0.1 mm to about 3 mm. Especially athin dip-formed article having a thickness of 0.1 to 0.3 mm can be made.

Thus the dip-formed article of the present invention having the abovecharacteristics is suitable for, for example, a nipple of nursingbottle, medical articles such as a dropper, a duct and a water pillow;toys such as a balloon, dolls and a ball, and sporting goods such as aball; industrial articles such as a pressure molding bag and a gasstorage bag; unsupported gloves and supported gloves for surgical,household, agricultural, fishery and industrial uses; and a finger cot.The dip-formed article is especially advantageously used as thin glovessuch as thin surgical gloves.

1. A dip-forming latex obtained by copolymerization of 50 to 89.5 partsby weight of a conjugated diene monomer, 10 to 40 parts by weight of anethylenically unsaturated nitrile monomer, 0.5 to 10 parts by weight ofan ethylenically unsaturated acid monomer and 0 to 20 parts by weight ofother copolymerizable ethylenically unsaturated monomer, provided thatthe total of these monomers is 100 parts by weight, wherein saidcopolymerization is initiated with a monomer mixture comprising at least80% by weight of the amount of conjugated diene monomer used, at least50% by weight of the amount of ethylenically unsaturated nitrile monomerused, 10 to 90% by weight of the amount of ethylenically unsaturatedacid monomer used and at least 80% by weight of the amount of othercopolymerizable ethylenically unsaturated monomer used, and thereafter,the remainders of monomers are added to a polymerization system tocontinue copolymerization.
 2. The dip-forming latex according to claim1, which is obtained by copolymerization of 55 to 84 parts by weight ofa conjugated diene monomer, 15 to 36 parts by weight of an ethylenicallyunsaturated nitrile monomer, 1 to 9 parts by weight of an ethylenicallyunsaturated acid monomer and 0 to 15 parts by weight of othercopolymerizable ethylenically unsaturated monomer, provided that thetotal of these monomers is 100 parts by weight.
 3. The dip-forming latexaccording to claim 1, which is obtained by copolymerization of 65 to 81parts by weight of a conjugated diene monomer, 18 to 27 parts by weightof an ethylenically unsaturated nitrile monomer, 1 to 8 parts by weightof an ethylenically unsaturated acid monomer and 0 to 10 parts by weightof other copolymerizable ethylenically unsaturated monomer, providedthat the total of these monomers is 100 parts by weight.
 4. Thedip-forming latex according to any one of claims 1 to 3, wherein thecopolymerization is initiated with a monomer mixture comprising at least80% by weight of the amount of conjugated diene monomer used, at least80% by weight of the amount of ethylenically unsaturated nitrile monomerused, 10 to 90% by weight of the amount of ethylenically unsaturatedacid monomer used and at least 80% by weight of the amount of othercopolymerizable ethylenically unsaturated monomer used, and thereafter,the remainders of monomers are added to a polymerization system tocontinue copolymerization.
 5. The dip-forming latex according to any oneof claims 1 to 3, wherein the copolymerization is initiated with amonomer mixture comprising at least 90% by weight of the amount ofconjugated diene monomer used, at least 90% by weight of the amount ofethylenically unsaturated nitrile monomer used, 30 to 85% by weight ofthe amount of ethylenically unsaturated acid monomer used and at least90% by weight of the amount of other copolymerizable ethylenicallyunsaturated monomer used, and thereafter, the remainders of monomers areadded to a polymerization system to continue copolymerization.
 6. Thedip-forming latex according to claim 4 or 5, wherein, after thecopolymerization of the monomer mixture is initiated, the remainder ofethylenically unsaturated acid monomer is added while the polymerizationconversion of the total monomers added is within a range of 5 to 90%,and the remainders of conjugated diene monomer, ethylenicallyunsaturated nitrile monomer and other copolymerizable ethylenicallyunsaturated monomer are added at any time before the termination ofcopolymerization.
 7. The dip-forming latex according to claim 4 or 5,wherein, after the copolymerization of the monomer mixture is initiated,the remainder of ethylenically unsaturated acid monomer is added whilethe polymerization conversion of the total monomers added is within arange of 20 to 80%, and the remainders of conjugated diene monomer,ethylenically unsaturated nitrile monomer and other copolymerizableethylenically unsaturated monomer are added at any time before thetermination of copolymerization.
 8. The dip-forming latex according toany one of claims 1 to 3, wherein the copolymerization is initiated witha monomer mixture comprising at least 80% by weight of the amount ofconjugated diene monomer used, 50 to 90% by weight of the amount ofethylenically unsaturated nitrile monomer used, 40 to 90% by weight ofthe amount of ethylenically unsaturated acid monomer used and at least80% by weight of the amount of other copolymerizable ethylenicallyunsaturated monomer used, and thereafter, the remainders ofethylenically unsaturated nitrile monomer and ethylenically unsaturatedacid monomer are added while the polymerization conversion of the totalmonomers added is within a range of 5 to 95%, and the remainders ofconjugated diene monomer and other copolymerizable ethylenicallyunsaturated monomer are added at any time before the termination ofcopolymerization.
 9. The dip-forming latex according to claim 8, whereinthe copolymerization is initiated with a monomer mixture comprising atleast 90% by weight of the amount of conjugated diene monomer used, 55to 85% by weight of the amount of ethylenically unsaturated nitrilemonomer used, 50 to 85% by weight of the amount of ethylenicallyunsaturated acid monomer used and at least 90% by weight of the amountof other copolymerizable ethylenically unsaturated monomer used.
 10. Thedip-forming latex according to claim 8 or 9, wherein, after thecopolymerization of the monomer mixture is initiated, the remainders ofethylenically unsaturated nitrile monomer and ethylenically unsaturatedacid monomer are added while the polymerization conversion of the totalmonomers added is within a range of 10 to 90%, and the remainders ofconjugated diene monomer and other copolymerizable ethylenicallyunsaturated monomer are added at any time before the termination ofcopolymerization.
 11. The dip-forming latex according to any one ofclaims 8 to 10, wherein, after the copolymerization of the monomermixture is initiated, the remainder of ethylenically unsaturated nitrilemonomer is added while the polymerization conversion of theethylenically unsaturated nitrile monomer is within a range of 40 to95%.
 12. The dip-forming latex according to any one of claims 8 to 11,wherein, after the copolymerization of the monomer mixture is initiated,the remainder of ethylenically unsaturated acid monomer is added whilethe polymerization conversion of the total monomers added is within arange of 20 to 80%.
 13. The dip-forming latex according to any one ofclaims 8 to 12, wherein, after the copolymerization of the monomermixture is initiated, the remainder of ethylenically unsaturated nitrilemonomer is added at two or more times.
 14. The dip-forming latexaccording to any one of claims 1 to 13, wherein the conjugated dienemonomer is at least one monomer selected from the group consisting of1,3-butadiene and isoprene.
 15. The dip-forming latex according to anyone of claims 1 to 14, wherein the ethylenically unsaturated nitrilemonomer is at least one monomer selected from the group consisting ofacrylonitrile and methacrylonitrile.
 16. The dip-forming latex accordingto any one of claims 1 to 15, wherein the ethylenically unsaturated acidmonomer is at least one monomer selected from the group consisting ofethylenically unsaturated carboxylic acid monomers.
 17. A dip-formingcomposition comprising the dip-forming latex as claimed in any one ofclaims 1 to
 16. 18. The dip-forming composition according to claim 17,which further comprises a vulcanizer selected from the group consistingof sulfur and polyamines, and a vulcanization accelerator.
 19. Adip-formed article made by dip-forming the dip-forming composition asclaimed in claim 17 or
 18. 20. A dip-formed article having a tensilestress at 300% elongation of not larger than 2.5 MPa, a tensile strengthof at least 15 MPa, and a tensile stress retention of at least 70% asmeasured when 6 minutes elapses from the time of 100% elongation. 21.The dip-formed article according to claim 20, which has a swellingdegree in methyl ethyl ketone of not larger than 200%.
 22. Thedip-formed article according to claim 20 or 21, which is a glove.